FIG. 1A shows a conventional AC controller 200 having a silicon control rectifier (xe2x80x9cSCRxe2x80x9d) as the solid-state device switch. The controller 200 includes a mains or power source 202 that supplies power, a switch 202 that regulates the power, and a control circuit 204 that controls the turn on and off characteristics of the switch. A load 206 receives the power.
The switch 202 includes a first silicon control rectifier (xe2x80x9cSCRxe2x80x9d) 208 and a second SCR 210 that are arranged in an xe2x80x9canti-parallelxe2x80x9d formation to conduct currents in both directions. Like a diode, an SCR generally conducts currents in a single direction so two SCRs are provided in a reverse orientation to serve as an AC switch.
There are three basic control modes in SCR devices: (1) on/off, (2) zero-firing, and (3) phase-firing. The first mode or on/off mode is the simplest method and replicates the action of an electromechanical switch. The power is either turned on or turned off according to the commands of the control circuits 204. Generally, the device is xe2x80x9conxe2x80x9d if a command signal is applied to the SCR, and the device is xe2x80x9coffxe2x80x9d if the command signal is removed.
The second control mode or zero-firing mode switches the SCRs on and off, but provides a proportional control capability. With this control mode, the number of xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d AC cycles is varied to maintain a steady voltage level to the load while turning the power on and off. While effective, the zero-firing is not suited for some application. The voltage applied to the load is either zero or full because zero-firing turns the SCR either completely on or off. This is not suitable for certain exotic load elements, such as, molybdenum disilicide. Molybdenum disilicide""s resistance is nearly zero when cold, but it increases with temperature. A large current surge results each time the SCR is turned on from a cold state. These current surges can damage SCRs.
The third control mode or phase-firing provides infinite variable control of voltage being applied to the load, much like a light dimmer. Similar to the zero-firing, the phase firing provides timed gate pulses or command signals to the SCRs. The phase-firing mode, however, turns on each of the two SCRs in an AC switch only for a portion of the respective half-cycles.
Referring to FIG. 1B, the SCRs are being fired on in the AC cycle at a given angle xcex1, as described in the current and voltage waveforms. Once fired on, as long as there is a forward-on current flow iA1 (FIG. 1A), the SCR stays on. The SCR turns off as soon as the current iA1 decreases to substantially zero current level or below the threshold current level of the SCR. As illustrated, the voltage and current waveform is a function of the firing angle xcex1.
FIGS. 2A-2C illustrate the current and voltage waveforms of the AC controller 200 as a function of the firing angle xcex1 for a resistive load (FIG. 2A), for a resistive-inductive load (FIG. 2B), and inductive load (FIG. 2C).
Even at resistive loads, a firing angle xcex1 greater than 0, indicating that power consumption of the load is controlled by the AC controller, leads to the generation of first harmonic reactive power and of further harmonic content caused by the distortion of the current waveform. This creates high electromagnetic noise or xe2x80x9cEMCxe2x80x9d for the AC controller. A countermeasure step, therefore, is required to compensate the reactive power component to reduce the EMC. The current flow at firing angles xcex1 greater than 0, is intermittent on the mains side and on the load side. This also causes an increase of EMC in the AC controller 200.
Although the load and firing angle xcex1 determine the current waveform, there is no mechanism in the AC control circuit 200 to control them. The EMC generally is reduced in such circuits by adding passive filters. These added filters add to the cost, size and weight of the AC control circuits.
Embodiments of the present invention provides the capability of an AC switch to be turned on and off in any time during the AC cycle, allowing the user to apply any desirable pulse pattern to said AC switches, even turning them on and off multiple times within the AC cycle. Accordingly, inverters and converters can be developed with enhanced control features that reduce undesirable noise problems, improve dynamic response of the system to interference or to changing power demands, and improve power regulation and the efficiency of the system.
In one embodiment, a method for operating an alternating-current (AC) controller system includes providing a first bi-directional switch coupled to a load and an AC power source. The first bi-directional switch is a solid-state device. The first switch is turned on in a first half-cycle of an AC cycle. The first switch is turned off in the first half-cycle of the AC cycle.
In another embodiment, an alternating-current (AC) controller system includes a first switch including a reverse blocking insulated gate bipolar transistor (xe2x80x9cRIGBTxe2x80x9d) coupled to a power supply to regulate a current supplied by the power supply. The first switch is configured to be turned off while the current is flowing through the first RIGBT.
In another embodiment, a method for operating an alternating-current (AC) controller system including providing an AC switch coupled to a power supply and a load. The AC switch is turned on to supply a current to the load. The AC switch is turned off while the current is flowing through the switch and being supplied to the load.
In another embodiment, an AC controller includes an AC source having a first pole and a second pole, a load having a first node and a second node, and a first bidirectional switch, a solid state device, that is coupled to the first pole of the AC source and the first node of the load. The bidirectional switch has at least one reverse blocking insulated gate bipolar transistor (IGBT).
In yet another embodiment, a multi-phase switch system includes a first AC controller including an AC source having a first pole and a second pole, a load having a first node and a second node, and a first bidirectional switch being a solid state device coupled to the first pole of the AC source and the first node of the load. The bidirectional switch has at least one reverse blocking insulated gate bipolar transistor (IGBT). The system also includes a second AC controller including a second AC source having a first pole and a second pole, a second load having a first node and a second node, and a second bidirectional switch being a solid state device coupled to the first pole of the AC source and the first node of the load. The second bidirectional switch has at least one reverse blocking IGBT.
In yet another embodiment, a method for operating an AC controller includes providing an AC source having a first pole and a second pole; providing a first load having a first node and a second node; providing a first bidirectional switch being a solid state device coupled to the first pole of the AC source and the first node of the first load, wherein the bidirectional switch has at least one reverse blocking IGBT; and controlling the first switch to adjust a power factor for optimal performance of the AC controller with respect to the AC source.